A helmet is provided including an attachable and detachable shield plate by a simple structure. The helmet includes a helmet main body; a first plate positioned onto an outer surface of the helmet main body; a second plate structurally combined with the first plate and capable of sliding between a locking position and an unlocking position with respect to the first plate; a spring for pressurizing the second plate toward the locking position; and a shield plate capable of rotating with respect to the helmet main body and attachably and detachably installed on the first plate by the slide movement.